Manufacturing process and apparatus for making a helical rib tube

ABSTRACT

A method of making a composite tubular article made up of at least two concentric members, the outer of said members being tubular and the inner of said members being at least cylindrical and preferably tubular, and struts disposed in supporting and positioning orientation between the inner and outer members, by extruding a moldable material into the desired profile; stretching the inner tubular member over a cooling mandrel and disposing a cooling sleeve over the outer tubular member under conditions sufficient to solidify the article; twisting and longitudinally pulling the solidified article, so that it rotates over the cooling mandrel and inside the cooling sleeve while it is moving down stream across the cooling mandrel, whereby causing the extrudate to be twisted into a helical shape.

[0001] This application is a continuation in part of application Ser.No. 09/626,886 filed Jul. 27, 2000, now U.S. Pat. No. 6,405,974 whichwas a continuation in part of PCT Application Number PCT/US99/17172,filed Jul. 29, 1999 which was a continuation in part of ProvisionalApplication Serial No. 60/096,237, filed Aug. 12, 1998 and ProvisionalApplication Serial No. 60/101,935, filed Sep. 25, 1998. This applicationis a continuation in part of Provisional Application Serial No.60/309,494, filed Aug. 3, 2001.

BACKGROUND OF THE INVENTION

[0002] This invention is related to a method of making certain kinds oftubing. It more specifically refers to a method of making a multiplewalled tube, especially a two concentric walled tube with the two wallsbeing radially spaced apart and having rib members in supportingrelationship between and attached to the walls. The rib members of thecomposite tubular structure of this invention have been referred to asribs, webs, struts and other identifying names. All of these members areintended to be included in this invention regardless of the name bywhich they are called. For ease of description and understanding, thisinvention will be described in relation to a two concentric walled tube.It will be understood that tubes with more than two concentric walls areadapted to be made according to this invention. Further, although theinstant description generally refers to hollow, multi-walled tubes, itshould be clear that multi-wall tubes with a solid inner element can bemade according to this invention. Still further, it should be clear thatthe manufacturing method and apparatus described herein could be used tomake a multi-walled, hollow core tube with an armature residing in thehollow inner tube. The armature can be rigid or flexible and may be madeof a different material than the multi-walled tube. It can be wholly orpartially in contact with the interior wall of the inner tube.

[0003] Core members are widely used in the plastic film industry as wellas in the carpet and other textile fabric industries. Rolls of materialare wound around suitable cores for shipping and as a convenient formfor the material rolled thereon to be used as feed to downstreamoperations for processing the material rolled onto the core. It iscommon for these large rolls of material to be wound quite tightlybecause if the volume of the rolled material is minimized, shippingcosts are also minimized. It is also a fact that as material is wound ona core, it is under at least some tension. Therefore, when the roll iscompleted, the rolled material tends to collapse upon itself therebyexerting more or less crushing force against the core. In addition,shrink or stretch wrap film is also conventionally wound on cores. Thistype of film was made with at least a unidirectional stretchingoperation and it therefore has a strong tendency to shrink, that is, thefilm has a memory wherefore its length (and sometimes its width as well)always tries to shorten itself. This tendency causes an even greatercrushing stress on the core around which the shrink wrap film is wound.

[0004] It has been conventional to use disposable cardboard coremembers. However, it has been found that, although cardboard is veryinexpensive and that is a very strong recommendation for its use, itdoes not have a crushing strength that is sufficient for it to withstandhigher inwardly directed forces that are inherent in some woundmaterials, especially wound plastic film.

[0005] In order to rectify this situation, the industry has used metal(hollow or solid), wooden and/or solid plastic cores. Some thought hasbeen given to using hollow plastic cores but it has been found that thecores must be exceptionally thick so as to be able to withstand theinwardly directed radial pressure that the wound material exerts on thecore. Therefore, so much plastic must be used to make a core of thedesired crush resistance, that it is uneconomical.

[0006] In the prior art, core elements have been used that consist of asolid material, such as in the form of a solid cylinder. Alternatively,cores are known that are in the form of a hollow cylinder having asingle, relatively thick wall. Cores of this configuration, for examplemade of metal or plastic materials, can have excellent crushingstrengths. The disadvantage of such cores is that they are generallyquite expensive. If the user returned the cores after use, the initialexpense of purchasing the cores would not be such a detriment becausethe shipper/seller could then reuse the returned cores. The cost of thecores would then be amortized across multiple uses rather than a singledisposable use. However, users have not generally returned the usedcores. The manufacturer can only amortize the cost of the core against asingle use and thus must add the cost of the cores into his sellingprice of the material that is wound about the core. This substantiallyincreases the price at which the wound material must be sold wherebymaking it less competitive. All in all, these prior art plastic coreshave not met with any substantial degree of success and there is needfor improvement in this technology.

[0007] Recently, a modified tubular core structure has been inventedthat has sufficient crushing strength to be useful in industrialapplications, and employs little enough material to be economicallyviable. This tubular core is sufficiently inexpensive so that it doesnot add a significant amount to the cost of the material wound on thecore wherefore it does not have to be returned to the seller. This corematerial is made up of concentric tubular members (preferably 2) withrib members disposed there between and in supporting relationship toboth tubular members. The interior element of this core structure can besolid or hollow. Tubes with more than two concentric elements arecontemplated.

[0008] Other uses have been found for this novel tubular structure in aninternally hollow configuration. These structures find application inmany industries such as: agriculture, construction, irrigation, waterand sewer distribution, telecommunications and other electrical conduitmarkets. While smaller diameter multi-walled tubes have use as windingcore elements, larger diameter multi-walled, hollow, tubular structuresare useful in drain pipe and culvert applications. A further aspect ofthis invention, then is the manufacture of larger diameter,multi-walled, hollow, composite structures that are both relativelylight in weight and strong in flex and crush strengths.

[0009] It is interesting to note that the material from which thetubular structure of this invention is made can be varied depending onthe use to which the structure is to be put. For use as a core uponwhich sheet-like materials are to be wrapped, the composite tubularmember should be relatively stiff and not either flexible or crushable.For culvert applications, the tubular structure should have excellentcrushing strength but also may have sufficient longitudinal flexibilityto be able to be bent around structures that are encountered in theground, such as large boulders. For drain pipe and septic systemapplications, the tubular structure can be quite flexible, althoughgenerally, the flexibility of the pipes of the instant invention issomewhat less than conventional corrugated pipe. Further, for drain pipeand septic field applications, the structures of this invention can beperforated. The multi-walled tubular product of this invention hasunique application in its perforated form. Ground water draining throughthe outer wall will be directed into channels (preferably helicalchannels, that exist between the inner and outer walls and then willdrain to the end of the tube without necessarily penetrating through theinner wall and into the interior hollow structure. This structureenables the inner tubular element to be solid and therefore much moresupportable. Of course, it is considered to be an embodiment of thisinvention to utilize the products made by the manufacturing process andapparatus of this invention as drain pipes with perforations throughboth the inner and the outer tubes.

[0010] This tubular structure to which this invention is directedcomprises a plurality of elongated tubes, preferably of extrudablematerial such as plastic (e.g. polystyrene, polyethylene or the like) oraluminum or other metals. The tube is made up of multiple concentrictubular members that are radially spaced from each other. It isanticipated that the concentric tubes will be coextruded.

[0011] At least one, but preferably a plurality, of radially directedrib(s) is disposed between and in supporting relationship to theconcentric tubular members. The rib(s) serves to maintain the relativeradial positions of the concentric tubular members, and to providesubstantial crushing resistance without adding significant amount ofextra material. In many applications, the rib will be substantiallynormal to the tangent to the interior and exterior tubular members.Although struts that are disposed normal to the surfaces of theconcentric tubes are suited to this use, it has been found that evengreater strength increase and weight minimization are achievable if theribs are disposed at an angle, other than right, with respect to thetangents of the interior and exterior tubular elements so as to formgenerally triangular rib members.

[0012] Next adjacent ones of these angularly disposed supporting ribelements are suitably in contact with each other at one end thereof.Thus, the two ends of at least some, but preferably all, of theangularly disposed supporting rib members are in supporting contact withthe outside surface of the inner tube, and the inside surface of theouter tube, respectively. At, or very near, the line along which theangular supporting rib member is in contact with either the inner or theouter tube, each angular supporting member is also preferably in contactwith the next adjacent angularly disposed supporting rib member. Thus,if the two adjacent angularly disposed supporting rib members are incontact with each other, they, together with the tubular wall that isopposite to their contact point, form a generally triangular crosssection longitudinal rib member.

[0013] If the next adjacent angularly disposed supporting rib membersare spaced a small distance apart at the location where they contact therespective outer and inner walls of the concentric tubular members,respectively, the assembly of two next adjacent support members and twoopposite segments of the concentric tubular wall elements form a trussassembly that has a cell or cavity that is generally trapezoidal incross section. It has been found that if the length of the shorter wallsegment of one of the tubular elements is relatively short, for someunexplained reason, composite tubes with rib members having such atrapezoidal cross section are stronger than the those with triangularlyshaped rib members. In either case, it is preferred, but not absolutelyrequired, that the rib members extend the entire length of the tubulararticle. These rib members should preferably have a circumferential aswell as a longitudinal component to their direction.

[0014] It has also been found preferable to dispose a plurality of ribmembers about (preferably evenly about) the entire cylindrical spacebetween the inner and outer tubular members. Suitably, the dispositionof the rib members should be symmetric about the outer and inner,respectively, circumferences of the tubular members. It has been foundto be most preferred to have a plurality of rib members disposedthroughout the entire cylindrical space between the inner and the outertubular members with each adjacent rib member sharing a wall with itsnext adjacent rib member.

[0015] In the preferred embodiment of this structure, the rib membersare disposed in a longitudinally helical configuration. This structureimparts excellent crush resistance because it provides a component ofstiffness that is both tangential and radial to the walls, and it alsoprovides the lightness that is characteristics to this product. Further,in appropriate situations, and with the correctly designatedconstruction material, the helical configuration of the rib members addsa significant amount of longitudinal and radial bending stiffness. Theangle that the helix makes to the longitudinal axis of the instantproduct is a determinant of the amount of bending stress can bewithstood (that is how radially stiff the product is).

[0016] An interestingly adjunct of this structure is the fact that theprocess of forming the ribs/trusses into helical configurationunexpectedly increases the degree of roundness of the final product.Thus, the formation of this helical rib configuration causes the crosssection of the composite tubular product to remain substantiallyconstant and symmetrical. That is, if the concentric tubular memberseach have a circular cross section, the helical configuration of the ribmembers tends to cause the entire article to have a circular crosssection and minimizes distortion of the circular cross section into anelliptical or oblate cross section, or a cross section with hills andvalleys.

[0017] It will be appreciated that making hollow tubes of thisconfiguration on a commercial scale is a very difficult undertaking. Theinstant invention is directed to a particularly effective method ofmaking these articles and apparatus suited for carrying out this method.

OBJECTS AND GENERAL STATEMENT OF THE INVENTION

[0018] It is an object of this invention to provide an efficient methodof making elongated tubular articles that comprise at least twogenerally concentric, cylindrical members, with rib members disposedbetween, and in a supporting relationship to, the cylindrical members.

[0019] It is another object of this invention to provide a method ofmaking such structures from extruded material such as plastic materials.

[0020] It is a further object of this invention to provide an improvedmethod of making a multi-concentric walled article with rib supportingmembers disposed in longitudinally as well as circumferentialrelationship between next adjacent concentric elements.

[0021] It is a still further object of this invention to provide amulti-walled tubular structure of surprisingly improved strengthcharacteristics as well as an improved method of making such and animproved apparatus for carrying out the improved method.

[0022] Other and additional objects will become apparent from aconsideration of this entire specification, including the drawingshereof.

[0023] In accord with and fulfilling these objects, one aspect of thisinvention resides in a method of forming a multi-wall tubular articlecomprising:

[0024] feeding at least one meltable and extrudable material into atleast one extruder;

[0025] melting the material(s) into flowing fluid form;

[0026] providing a plurality of radially spaced, substantially circularextrusion dies adapted to extrude a corresponding plurality of fluidform streams of molten material, with at least one stream disposed aboutthe periphery of the extrusion die, where an outer stream is radiallyseparated from the next most inner stream;

[0027] extruding at least two generally concentric, substantiallyendless cylindrical members where at least one of said members is anouter tubular member and another of the members is an inner cylindricalmember;

[0028] extruding rib members, comprising angularly directed (withrespect to the tangents to the surfaces of the concentric members)struts, disposed between the inwardly directed surface of the outertubular member and the outwardly directed surface of the innercylindrical member, and in at least partial contact with the facingsurfaces of the tubular members, to form a composite structure with ribmembers disposed in supporting relationship between the innercylindrical and the outer tubular members;

[0029] wherein next adjacent ribs are disposed at least proximate toeach other where they contact a surface of one of the tubular members;

[0030] wherein next adjacent ribs, together with the surfaces of thetubular members in which they are in contact, form generally triangularor trapezoidal enclosures (cavities);

[0031] admitting a gas through the extruder die into the triangular ortrapezoidal cross section areas of the extruded composite structurewhereby causing spaces between strut members to be substantially free ofextruded material;

[0032] where the inner cylindrical member is a tubular member, drawingthe interior tubular member, of the substantially endless extrudedcomposite structure, over a cooling drum, that, in addition to providingcooling to the interior surface of the composite tubular member, hasopenings that through which a vacuum (including a pulsed vacuum) isapplied in an amount and pulse frequency sufficient to draw the innerdirected surface of the inner tubular member against its surface;

[0033] wherein the surface of the cooling drum may be tapered in adownstream direction such that, as the inner tubular member cools andthereby shrinks, the diameter of the cooling drum proportionally reducesan amount sufficient to permit the shrunk tubular structure to readilymove across the cooling surface of the cooling drum and an amountsufficient to maintain cooling proximity between the inwardly directedsurface of the inner tubular member of the extrudate and the coolingsurface of the cooling drum, and thereby cooling and solidifying theinner tubular member and at least portions of the strut members that areproximate to the inner tubular member;

[0034] moving the cooling extrudate through a sizing sleeve andindependently applying a vacuum along with a cooling means, such aswater mist, onto the outwardly directed surface of the outer tubularmember of the extrudate under conditions sufficient to cool and solidifythe external tubular member as well as at least portions of the strutsthat are proximate to the outer tubular member;

[0035] wherein the combination of the outer vacuum cooling means and theinner cooling drum are sufficient to cool the inner tubular member andthe outer tubular member to substantially solid conditions, andsufficient to cool the struts an amount that renders them shape stable;

[0036] drawing the cooled, solidified, multi-walled tubing away from thecooling operation while simultaneously twisting the external andinternal tubular members, about their longitudinal axis, wherebyconverting the ribs from their extruded longitudinal orientation to anorientation that has both a longitudinal as well as a circumferentialorientation, preferably into a helical orientation; and

[0037] if desired, cutting the thus made multi-walled composite tubingwith internal struts into desired lengths.

[0038] According to this invention, the cooling drum should be made aslong as practical, in order to apply sufficient cooling at a rate thatwill retain the integrity to the inner tubing. In order to prevent theextrudate from freezing on the cooling drum during start-up before ithas progressed to a normal operating speed, the cooling drum may bedisposed on a suitable longitudinally moveable carriage. In one aspectof this invention, the longitudinal carriage and the extrusion die (thatis adapted to extrude concentric tubular members) are so related in sizeand shape that the cooling drum can be retracted into a passage throughthe center of the extrusion die.

[0039] This structure will allow the cooling drum to be partiallywithdrawn into the passage in the die, so that during start-up only thedownstream portion of the drum extends beyond the die. This enables themolten extrudate to be pulled over the cooling drum and guided into thedownstream processing before it has the time to solidify and freeze onthe cooling drum during start up. As the operation proceeds, the coolingdrum can be moved in a downstream direction, that is, extending fartherdownstream of the extrusion die so that cooling of the interior tubebecomes sufficient to solidify the inner tubular member during lined outoperation of the instant method. That is, after startup, and when thesystem is running at lined out speeds, sufficient cooling takes place onthe cooling drum to solidify the tubular structure which is then pulledoff the drum. Because the system is running at lined out speeds, eventhough the interior tubular member is solidified on the cooling drum, itdoes not stick to the drum.

[0040] Other means of starting up the line can be used as well. Theimportant thing is that the extrudate be cooled by contact with thecooling drum at a rate such that the inner tubular member is solidifiedbut not frozen to the cooling drum. In this regard, it should be pointedout that one of the downstream processes that the extrudate will besubjected to is a pulling and twisting operation. The twisting componentof this downstream operation twists the extrudate as it emerges from theextruder die in order to instill a circumferential component to the ribmembers. Thus, the extrudate must be sufficiently solid as it leaves thecooling drum that the twisting operation acts through the solidifiedextrudate back to the exit of the extrudate from the extruder die wherethe extrudate is still moldable. It will be clear, therefore, that thecooling drum must either be rotatable about a longitudinal axis, or thecooled and solidified extrudate must be able to rotationally andlongitudinally slide over the surface of the cooling drum.

[0041] According to a preferred aspect of this invention, the coolingdrum is rotatable about its streamwise axis. It has been found that inorder for the structure to provide the maximum stiffness, the rib cellsshould be as symmetrical as possible. It has been found to beadvantageous to provide rotational capability to the inner cooling drum.This enables the operator to have much better control over the relativerotation of the inner tubular element as compared to the outer tubularelement. It is considered to be within the scope of this invention torotate the inner tubular member at a different rotational speed than itsnext adjacent outer tubular member to accommodate the fact that theinner tubular member has a smaller diameter than the next outer tubularmember. However, adjustability of the relative rotational speeds of thetubular members is a feature of this invention and so gives the operatorexcellent control.

[0042] Molten resin from an extruder is fed through a very complex die,which causes the extrudate to form the cross-sectional shape of thecomposite tubular structure of this invention. The extrudate is cooledand sized within a cooling tank and over a mandrel by simultaneouslyapplying cooling from both outside and inside; the inside wall is cooledand thereby solidified by an internal drum/mandrel, and the outside wallis cooled and solidified by a sizing sleeve and a cooling bath of fluid,such as a water spray.

[0043] A puller draws the multi-walled tube away from the coolingsection of the instant apparatus while simultaneously twisting it aboutits longitudinal axis. In a preferred embodiment, the combination ofpulling and twisting gives the tubular product a helical motion thatcauses the rib members (as well as the inner and outer tubular members)to be twisted into a helical shape. This is most desirable for severalreasons. First, helical ribs provide both longitudinal andcircumferential support to the plural tubular members of the product ofthis invention. Second, twisting the tube while it is still somewhatmoldable tends to smooth out possible imperfections in the roundness ofthe tubular product.

[0044] It is considered to be within the scope of this invention toapply a circumferential component to the ribs in a manner other than bymaking them helical. The twisting motion can be reciprocating to therebyapply a sinusoidal shape to the ribs. Other shapes that will incorporatea circumferential as well as a longitudinal component to the rib membersare applicable as well. Since the extrudate is always moving in astreamwise direction, it is difficult to cause the ribs to have acircumferential orientation without having a longitudinal orientation aswell. The ratio of the circumferential orientation to the longitudinalorientation of the rib members determines the relative bending and crushresistance of the multiwall product of this invention. It will alsodetermine the weight per unit of length of the product. Increased crushresistance is achieved when the angle intercepted between thelongitudinal axis and a tangent to the ribs is high, however, thisresults in a lower resistance to longitudinal bending. Conversely, ifthis angle is small, there is greater longitudinal stiffness but lowercrush resistance. A proper balance must be struck for each applicationof the instant method and product.

[0045] If desired, the final product can be cut to any desired length bya special cutter that preferably leaves no rough edges. The cut piecescan be stacked by a stacking table. Details of each of these operationsare described below. It is within the scope of this invention to coilthe final product if the cut lengths are long enough to warrant it. Itis also within the scope of this invention to perforate one or more thanone of the concentric tubular members so that the product can permitground water to seep into the tube and be drained away or to permitliquid in the inner tube to pass through the tube walls.

BRIEF DESCRIPTION OF THE DRAWING

[0046]FIG. 1 is a schematic view of an operating line of the apparatusused in this invention;

[0047]FIG. 2 is a sectional view of an extrusion die that is usedaccording to this invention;

[0048]FIG. 3 is a sectional view of the opening in the exit of the die;

[0049]FIG. 4 is an isometric view of a resin feed distribution means;

[0050]FIG. 5 is an exploded view of a pulsed vacuum cooling apparatusthat is useful in this invention;

[0051]FIG. 6 is a side view of a conventional puller apparatus thatserves to pull extrudate from the cooling system.

[0052]FIG. 7a is a cross section of a product embodiment of thisinvention showing a desired cross sectional configuration of the ribmembers hereof;

[0053]FIG. 7b is a cross section of a product embodiment of thisinvention showing an undesired cross sectional configuration of the ribmembers hereof that has been caused by improper rotationaldifferentiation between the rotation applied to the inner tubular memberand the outer tubular member;

[0054]FIG. 8a is a top view of double element puller that imposes ahelical twist to the rib members;

[0055]FIG. 8b is a side view of the puller of FIG. 8a;

[0056]FIG. 9a is a top view of a single element puller that imposes animproved helical twist to the rib members;

[0057]FIG. 9b is a side view of the puller of FIG. 9a;

[0058]FIG. 10 is a transverse sectional view of a cutter assembly forcutting tubular assemblies of this invention into the desired lengths;

[0059]FIG. 11 is a transverse sectional view of an additional feature ofan improved cutter assembly according to this invention;

[0060]FIG. 12 is a transverse sectional view of a rotational assemblyassociated with the cutter assembly;

[0061]FIG. 13 is a schematic perspective view of an improved pullerapparatus;

[0062]FIG. 14 is a schematic perspective view of a modified improvedpuller apparatus.

[0063]FIG. 15 is a perspective view (partially cut away) of a pipehaving perforation in the outer tubular member according to thisinvention; and

[0064]FIG. 16 is a perspective view (partially cut away) of a pipehaving perforation in the inner tubular member according to thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0065] Referring now to FIGS. 1, 2, 3 and 7 a, an extruder 10 is fedwith a suitable material, such as a plastic resin, 12. The resin ismelted in the extruder and passed through an extrusion die 14 ofsuitable configuration to form an extrudate 22 of appropriate size andshape. The extrudate 22 is a complex structure that is made up of aninner tubular element 16, an outer tubular element 18 and a plurality ofstruts 20 a and 20 b (see FIG. 7a) that are preferably, but notnecessarily, angularly disposed. The extrudate structure 22 is passedthrough a vacuum chamber 28 in which it is pulled over a sizing sleeveand cooling drum 24 while being passed through an external coolant, suchas a water spray, 26. Upon cooling and solidification, the extrudateproduct 22 is then pulled away from the cooling step and longitudinallytwisted by a puller/twister 30 (see FIGS. 6, 8a, 8 b, 9 a, 9 b, 13, and14) such that the rib members assume a configuration that has acircumferential as well as a longitudinal component, such as a helix.Note that air 32 is forced to flow between the two sets of die lips 14 aand 14 b (see FIG. 2) such that it enters the space between the ribmembers and keeps this space open and substantially free from beingclogged by excess extruded plastic material. Along with being pulled andtwisted, the extrudate 22 becomes fully solidified such that its ribmembers are in a substantially permanent shape (preferably helical). Itis then cut to suitable lengths by a cutter 34 and stacked at 37. Theproduct extrudate 22 may be coiled if it is of a long enough length.

[0066] As part of the manufacturing procedure described herein, meansmay be provided to perforate the extrudate 22 either after it has fullysolidified, or while it is still in a moldable condition. It ispreferred, but not essential, that such perforations be imparted afterthe multi-walled tubular article of this invention has cooled to anextent that it is no longer moldable. Reference is made to FIG. 15 for aperspective view of a perforated multi-walled tubular article 150. Notethat in a portion of the depicted article 150, the perforations 152 areonly through the outer tubular member whereby admitting fluid to enterinto the space between the several walls of the multi-walled tubulararticle 150 and then flow through the internal void space 35 defined byadjacent rib members and their attendant portions of the tubular memberwalls 20 c. Perforations may also or alternatively be positioned in theinner tubular portion of the instant article. It is within the scope ofthis invention to provide means to join the perforations in the innerand outer tubular members and to thereby provide a passageway for fluidto pass from the inner tubular volume out of the multiwalled tube ofthis invention or from the volume outside the outer tubular member intothe inner volume.

[0067] It should be clear that this invention is not limited to a methodand apparatus of producing a multi-walled tubular product that is madeup of only two tubular members. More than two concentric members can beemployed. Thus, with sufficient extruder capacity feeding sufficientextrusion die(s), three or four or even more concentric tubular memberscan make up the final composite multi-walled tubular structure. Wheremore than two tubular members are being created, the space between eachpair of tubular members is intended to be occupied by strut/ribelements, suitably helically shaped rib elements. Where there are morethan two tubular members, the ribs between any given pair of tubularmembers may be formed into trusses of the shape previously describedwhile the struts between other pairs of tubular members may be disposedsubstantially normal to the tangents of the surfaces of the tubularmembers with which they are in contact. The struts between any pair oftubular members may be aligned with the struts, or ribs, that aredisposed between the next adjacent pair of tubular members, or, in thealternative, they may be disposed intermediate between the struts/ribsin the next adjacent pair of tubular members.

[0068] It is considered to be within the scope of this invention toprovide, as the innermost tubular member, a solid (rather than hollow)cylinder. This solid cylinder will be connected with the next adjacentouter tubular member through suitable shaped struts and/or ribs asaforesaid. Preferably, the struts/ribs will be helically shaped. If asolid inner tubular member is employed, perforations will only bethrough the outer tubular member.

[0069] The particulars of this invention will be described withreference to using a single plastic material, such as polystyrene, asthe material of construction. It should be understood that this is byway of example and is not considered to be a limitation on the materialsthat can be used to practice this invention. Any moldable material canbe used, whether it is plastic or metal or glass or anything else. Forease of description, and because the nature of the individual moldablematerials is not a significant limitation on the instant invention, allof these moldable materials for use in this invention will behereinafter referred to as “resins” even though it is possible that someof the suitable moldable materials may not be plastic. It is important,however, that the possibility of using different materials for differenttubular members is encompassed by this invention. If multiple resins areemployed, different extruders will have to be used to feed different dielips. This technology is per se well known in the art. It is also withinthe scope of this invention to provide that the truss walls are the samematerial as one of the tubular members, but is also within thisinvention that the material that makes up the rib member struts isitself different from the material(s) that make up the tubular members.

[0070] A suitable plastic resin is melted and pumped by a sufficientnumber of conventional extruders 10, usually one extruder if there isone resin being employed, to feed moldable material, suitablythermoplastic material, to the extrusion die 14. This machine isconventional and is used universally in the plastics industry. Thenumber of extruders that need to be employed is a function of the crosssectional size of the composite tubular product that is being made, aswell as the number of different tubular members that are being made, aswell as the number of different resins being extruded. It is also afunction of the complexity of the cross section of the finished product.If the structure is to be composed of more than one type of resin, aseparate extruder will be required for each resin material. An exampleof a generic extruder that is suitably used in this invention is shownin FIG. 1.

[0071] The molten plastic is shaped into the desired cross section by aspecialized die 14, shown if FIG. 2. A sectional view of the extrudate22 is shown in FIG. 7a. What is shown as hatched areas in FIG. 3 is theshape of the opening in the die, with metal parts occupying the spacethat is not open and is shown in clear on the drawing. It should beclear that the die body 13 a surrounds a hollow outer ring 14 a that isadapted to form the outer surface of the outer tubular element 18 thatcreates the outside circumference of the multi-walled tubular product22. The die body 13 b is also disposed within the cylindrical inner ring14 b and is adapted to form the inner surface of the inner tubularelement 16. Portions of the die body 13 c are also disposed in the areasthat will become the open areas 14 c between the struts.

[0072] Molten resin will be forced between these die body parts 13 c toform the struts 20 a and 20 b. Additionally, molten resin will be forcedbetween these die body parts 13 c and the outer die body 13 a to formthe outer tubular member 18. Further, molten resin will be forced toflow between the outer surface of the inner die body part 14 b and thedie body parts 13 c to form the inner tubular element 16. Air 32 iseither forced, or simply allowed, to flow through the void space 14 cbetween the die lips 14 a and 14 b, in order to make sure the moltenresin that is being extruded to form the struts 20 a and 20 b does notflow together and block up the void spaces between the struts.

[0073] The cross section of the molten plastic as it leaves the die isshown as in FIG. 7a. It initially conforms to the shape of the die toform an inside tube 16, an outside tube 18, and multiple struts 20 a and20 b. The struts 20 a and b are preferably each oppositely angularlydisposed with respect to tangents to the surfaces of the inner and outertubular members 16 and 18, respectively. The struts 20 a and b arejoined to the inner surface of the outer tubular member 18 and to theouter surface of the inner tubular member 16 in a slightly spaced apartalignment so as to, together with the portions of the inner and outertubular member, form oppositely directed trapezoidal ribs supporting theinner and outer tubular members.

[0074] As discussed previously, in the die cross portion shown in FIG. 2and in the overall apparatus shown in FIG. 1, the molten resin exits tothe right. A typical (conventional) extrusion die would be fed from thecenter at the left, for best symmetry. But in the instant inventedmethod and apparatus for making multi-walled hollow tubular articles, itmay be important to have a hole 38 through the center of the die toaccommodate an internal cooling drum 24 (to be described later).Therefore in the die of this invention, the molten plastic can be madeto enter the die from one or more sides.

[0075] Distribution chambers 21 a, 21 b, 21 c and 21 d, such as shown inFIG. 4, provide uniform flow of molten plastic around the periphery ofthe die 14. This is accomplished by dividing each inflow of molten resin40 into two, suitably equal, parts 42 and 44, then, in a subsequentdistribution chamber (not shown but that is substantially identical tothe shown distribution chamber except that it has more chambers)dividing each of these streams 42 and 44 into two, suitably equal,parts, continuing in like manner forming increasing numbers of flowchannels, preferably substantially identically sized and shaped, andpreferably symmetrically distributed about the circumference of theextrusion die, until the channels are sufficiently close together thatthe flow of molten resin entering the final die orifice will besubstantially uniform and will fill the entire die extrusion area withequal quantities of resin. FIG. 4 is a view looking, in an upstreamdirection, at a distribution ring that divides each of four (4) inletstreams 40 into a total of eight (8) outlet streams 42 and 44. In oneexample of the die of this invention, the diameter of the exit flow isabout 3 inches, and this exit flow is fed by sixteen (16) flow channels.As can be seen in the FIG. 2, one embodiment of this invention employsthe three successive plates 20 a, 20 b and 20 c that have 2, 4, and 8outlet (see elements 42 and 44 in FIG. 4) holes, respectively, and themain die body has 16 outlet holes, each spaced about ⅝ inch apart. Thus,this arrangement enables a single stream of resin to be divided intosixteen (16) resin streams for more uniform distribution of molten resinabout the periphery of the extrusion die.

[0076] When the molten plastic first leaves the die during linestart-up, it is all compressed together so there are no open cells (openareas between the struts) within the cross section. As this materialmoves away from the die there is no air within these spaces, so vacuumforces the molten plastic together to close up the would be void areasbetween the struts and the inner and outer tubular members. In order toproduce open passages, it is necessary to cause air to reach the voidareas between the struts as they leave the die, keep the struts apartand keep the inner and outer tubular members from slumping until theextrudate solidifies. To accomplish this, holes 32 are provided throughthe metal of the die into each of the open spaces between the struts.Each of these holes communicates with atmospheric air or with a suitablepump, to force air into the space between the struts and insure that thespace between the struts remains open. The location of these holes 32 isshown in the sectional view of the die shown in FIG. 2.

[0077] There is a hole 38 through the center of the die 14 that isadapted to accommodate a cooling drum (to be described later) passingthere through (see FIG. 2). As a result, the molten plastic flowsdiagonally inward along a conical path, so that its flow distributioncan take place within the structure of the die. In this way the moltenresin can exit the distribution system at the smallest diameter possiblewithin the limitations of the center hole.

[0078] The cooling of a single wall tube is conventionally accomplishedin a vacuum tank, with the heat of the molten resin being removed byeither spraying the surface with a water spray or immersing the hotmolten resin (now in the extrudate form) in a cooling liquid, suitablywater. Conventionally, the tube enters a vacuum tank, where it is cooledon its outside by sprayed or immersed water, while the tube is beingdrawn outwardly toward an external sizing sleeve (sometimes called acalibrator) through the use of vacuum applied through the sleeve. Thecooling sleeve may be a cylindrical member with internal grooves and/orradial holes to allow vacuum to reach the hot resin tubular extrudate.Alternatively, the sleeve may consist of a series of flat rings withspace between the rings to allow water spray to reach the hot tubeextrudate. The sleeve may be cylindrical or slightly tapered inwardly ina downstream direction. The tube shrinks as it freezes, and if thesleeve is properly tapered it will maintain close proximity to the tubeduring the cooling and shrinking process. In this manner, the final tubediameter is controlled more accurately.

[0079] In producing an inner tubular member of a multi-tubular article,the diameter of inwardly directed surface of the tubular member isusually the most critical dimension, because when the multi-walledtubular article of this invention is used as a core upon which softgoods are wound, the inner surface of the inner tubular member is whatinterfaces with the winding equipment. With a conventional single walledtube, the internal diameter cannot be controlled directly andindependently. Rather this diametral dimension is the result of twoother factors, the cooled and shrunk outside diameter of the outertubular member and the wall thickness of the tubular member.

[0080] According to this invention, after the molten multi-walledextrudate leaves the die, it is cooled from both the outside and theinside. Each wall is treated as if it was a separate thin-wall tube, andeach is cooled substantially independently of the other. The ribs orstruts are disposed between, and attached in supporting relationship to,the two tubular walls. These struts are cooled, and thereby solidified,by heat conduction through the tubular walls. In effect, heat from theentire structure is removed to both the inside and outside of themulti-walled tubular article, as compared to conventional profileextrusion where heat is removed only from the outside. Thissignificantly increases the total heat removal rate, making possible amuch faster extrusion line speed for a given cooling tank length.Further, this process allows independent control of both outside andinside diameters, so the critical inside diameter is controlleddirectly. The slack, if any, is taken up in the cooling struts, whichare only indirectly cooled through the inner and outer tubular members,respectively.

[0081] The technique of cooling the outer tubular element, and itsassociated portion of the intermediate struts, is substantially the samecooling process and apparatus as described above for conventionalextrudate cooling. In its preferred embodiment, cooling is accomplishedby means of a water mist spray, and sizing is accomplished by passingthe extrudate through a series of rings, each tapered slightly smallertoward the downstream end, between which a vacuum is drawn, whereby theouter tubular member is continually drawn against a decreasing diameterexternal sizing sleeve 27 (see FIG. 1).

[0082] The inside wall is cooled by an internal sizing and cooling drum24. The drum is also preferably inwardly tapered in a down streamdirection so that as the tubular article solidifies and contracts indiameter, the cooling drum reduces in diameter. Thus, the inner coolingdrum is sufficiently smaller at its downstream end to be able toaccommodate shrinkage of the solidifying resin extrudate 22 as it cools.The amount of inward taper varies depending on the type of resin that isbeing used. For example, a drum for cooling polystyrene may be taperedso that it is about 1% to 1.5% smaller at its downstream end than it isat its upstream end. A drum for cooling polyethylene may require alarger taper, for example about 2% to 3%, because polyethylene shrinksmore than does polystyrene as it cools and solidifies. If there is toolittle taper, the cooling resin may shrink onto the drum and tightlygrip its surface causing the whole line to jam. If the taper is toogreat the drum will lose contact with the hot plastic, and cooling willbe less efficient.

[0083] The taper for polystyrene is critical, because once solidified,polystyrene is very stiff and it is unable to tolerate interference.Polyethylene can tolerate some interference because it can stretch tosome extent and because it has a lower coefficient of friction againstmetal surfaces than does polystyrene. Therefore, the shrinking clearanceis not so critical for polyethylene.

[0084] The inner tubular plastic element is drawn against the coolingdrum by vacuum applied through the drum. This is achieved by drawing avacuum through a series of circumferential grooves spaced at intervalsalong the drum length. In a preferred embodiment these grooves are about0.060 inch wide and are spaced about 1 inch apart. It should beunderstood that these dimensions could be substantially altered andstill be within the scope of this invention.

[0085] Because the vacuum draws the plastic relatively tightly onto thedrum, the movement of the tubular product along the drum must overcomefriction with the drum surface. Any sort of interruption in the lineoperation could cause the movement of the tubular product over the drumto hesitate for long enough for the plastic to cool more than thatspecific portion of the drum was designed for. The shrinkage caused bythis unexpected additional cooling will increase the adherence of thetubular article to the drum to such an extent that the friction betweenthe drum and the cooling resin article cannot be reasonably overcomewithout breaking the plastic tube. The amount of pulling required bythis additional friction will often not overcome the friction causing ajam. To reduce this possibility, the vacuum drawn on the cooling drum ispulsed, suitably on the order of about 100 to 400 times per minute. Thepulse rate could be higher or lower. Within each cycle, vacuum isapplied for the majority of the time, and when it is shut off, there isa pulse of pressure applied through the drum, that forces the tubulararticle to be released away from the drum. Thus, the tubular article issubjected to alternating vacuum and pressure that tends to diminish theadverse effects of too rapid cooling and shrinking.

[0086] The vacuum pulsing can be accomplished by any convenientmechanism such as a rotating disk as shown in FIG. 5. The disk 50, thatis sandwiched between an upper plate 52 and a lower plate 54, rotates,suitably at about 100 to 400 RPM. Vacuum is drawn through the hole 56 inthe lower plate 54 while the hole 58 in the upper plate is operativelyengaged with the above described grooves in the cooling drum 24. Theseholes 56 and 58 are positioned to be aligned and directly opposite oneanother. When the slot 61 is between the holes 56 and 58 in a positionsuch that there is communication between these holes, vacuum is appliedto the surface of the cooling drum (cooling mandrel) 24. During thatpart of the rotation when the element 60 is disposed in a blockingrelationship between the holes 56 and 58, the vacuum drawn from thecooling drum is vented to the atmosphere through the upper hole 58,while the while the flow to the vacuum pump through the lower hole 56 issealed off by the solid bottom under the element 60.

[0087] When the vacuum to the drum is interrupted, because the pressurein that hole is less than ambient, there is a sudden rush of air drawninto the upper hole 58. The inertia of that rushing air momentarilyexerts outward pressure on the inwardly directed surface of the innertubular member through the vacuum grooves in the drum, thus relievingthe effect of the vacuum.

[0088] As the multi-walled tube 22 is drawn away from the die 14 andthrough the cooling mechanism, it is pulled downstream andsimultaneously rotated or reciprocated about a longitudinal axis, sothat the tube moves along a helical path or serpentine path. Thelongitudinal pull is exerted directly on the outside wall of the tubulararticle by the puller 70 (shown in FIGS. 6, 8, 9, 13 and 14), so thatthe tubular article 22 moves in a helical or reciprocating path that isprescribed by the combination of the rotational and longitudinal forcesbeing exerted on the tube 22 by the puller. The outside wall 18 isdriven by the puller 70, which in turn pulls the inside wall 16 throughthe ribs 20 a and 20 b, so the path of the inside wall 16 can varydepending on other factors such the direction of the frictional pullthat the drum exerts on the inside wall 16. If the inside wall 16 doesnot follow the same path as the outside wall 18, the result is adistortion of the cells between the ribs, as can be seen in FIG. 7b. Toencourage the inside wall 16 to follow the desired rotational angularpath, at least a portion of the internal cooling drum 16 can be knurledto correspond to the desired helical angle, making a “path” for theinside wall to follow thereby making it easier for the inside wall tofollow the prescribed angle. In effect miniature tracks are provided toguide the direction of travel of the inside wall 16.

[0089] The desired angle between the ribs and the outer surface of theinner tubular element is not the same as the angle between the ribs andthe inside surface of the outer tubular element. As an example, if theangle between the ribs and the inner surface of the outer wall is 45°,for each rotation of the tube, a spot on the helix must advancelongitudinally a distance equal to the outside circumference of theouter wall 18. Therefore, at this rib angle, a given point movescircumferentially the same distance as it moves longitudinally. Theinside must also advance longitudinally that same distance. However, forthe same intercepted angle of rotation, the spot on the inner tubularmember moves circumferentially a smaller distance because of thecircumference of the inner tubular member is smaller. This results in adifferent angle between the ribs and the outer surface of the innertubular element 16 than the angle between the ribs and the inner surfaceof the outer tubular member 18.

[0090] In practice, even with the knurled internal cooling drum 24, itis sometimes not possible to achieve cells that are totally uniform. Asa means of controlling this circumstance as the line is running, meansmay be provided to rotate the internal cooling drum 24 slowly in onedirection or the other as needed to regularize the cells. For example,if the distortion is as shown in FIG. 7b, the internal drum would berotated clockwise to correct the distortion. If the distortion is in theother direction, the drum will be rotated counterclockwise to correctthe situation.

[0091] Line speed is limited by the speed at which heat can be removedfrom the multi-walled tubular extrudate 22. This heat removal, in turn,is related to the length of the extrudate that is being cooled. Thelength of the outer tube 18 that is being cooled at any given time canreadily be increased by providing a longer vacuum cooling enclosure 28.Increasing the length of the inside cooling mechanism is more complexand much more difficult.

[0092] When starting up the line, the molten plastic extrudate 22 ismanually pulled away from the die and stretched over the cooling drum24. Once the cooled multi-walled tube advances past the end of theinternal cooling drum, it is attached to a startup tube that pulls itthrough the remainder of the line. As the internal cooling drum lengthis increased, it becomes more difficult to thread up the line becausethe plastic freezes onto the drum before it can be pulled past the end.This effectively limits the maximum drum length. However, a longer drumwould be very desirable because it would permit higher cooling rates.One way to overcome this problem according to this invention is byproviding means to move the inner cooling drum longitudinally. One suchmeans calls for mounting the drum on a longitudinally moveable carriagethat allows it to be moved longitudinally upstream toward the plane ofthe extrusion die, and even further upstream through a hole in thecenter of the die.

[0093] During start-up, a given length of internal cooling drum ispermitted to extend downstream past the end of the die effectivelyshortening the internal cooling drum. Once the line is up and running,the drum is advanced streamwise as the line is speeded up effectivelylengthening the internal cooling drum. This is a very importantimprovement that has been accomplished by this invention.

[0094] As with a conventional profile extrusion process, the product isdrawn away from the die by a puller. However, the function of the pullerof this invention differs from that of a conventional puller 70 in twoways:

[0095] first, it rotates the tube as it pulls the tube longitudinally.This is what produces the desired configuration of the ribs having alongitudinal as well as circumferential component; and

[0096] second, it exerts a uniform radial squeeze around the tube,forcing the tube inwardly in a uniform manner and maintaining the tubeat an acceptable degree of roundness.

[0097] A conventional puller, as shown in FIG. 6, drives the productstraight downstream away from the die, with no rotation. The drivingforce is achieved by the puller belts or rollers squeezing the extrudatebetween 2 to 4 driven rolls or belts 70 a and 70 b that move the tube 22in a downstream direction. Because of the friction on the cooling drum,that may be caused by difficulties of cooling the complex structure ofthe tubular extrudate of this invention, it may require more pullingforce to move this product downstream than a typical single walledtubular product that is cooled from the outside only. To achieve thenecessary driving force, the rolls or belts of a conventional puller 70may have to squeeze harder, sometimes even hard enough to distort theextrudate product 22.

[0098] Referring to the single belt puller shown in FIGS. 9a and 9 b,the puller for a multi-walled, helical-ribbed tube 22 of this inventiondrives the outer tubular member 18 by means of a belt 72 that is wrappedhelically around the tube 22. As the belt 72 is driven, it moves thewhole of the plastic tube 22 downstream while simultaneously rotatingit. The inner and outer walls as well as the longitudinally straightdisposed struts (ribs) 20 a and 20 b that have been produced by theextrusion die are then caused to form a helical path along the tube(while the tubular product is still in a moldable condition) because ofthe rotation of the outer and inner tubular members with respect to thedie. Because the belt 72 forms one complete wrap about the tube, itexerts a direct radial force that completely encircles the tube 22, sothere is no force tending to flatten or distort the tube.

[0099] One difficulty encountered by this operation is that the tensionon the belt 72 applies sideways forces 101 a and 101 b that tend to bendthe tube 22. This problem is solved, as shown in FIGS. 8a and 8 b, byapplying a second belt 74 intertwined with the first belt and wrappedaround the extruded tube 22. Both belts 72 and 74 exert longitudinal androtational forces in the same direction, but each is out of phase withthe other such that the sideways force that each applies to the tubularproduct is directly opposite to the sidewise force applied by the other.Thus the bending forces that each exerts is effectively countered by thebending forces exerted by the other. This is shown at the in FIGS. 9aand 9 b.

[0100] In the pullers shown in both FIGS. 8(a and b) and 9(a and b), thedepicted belt pulls the extruded tube 22 forward (that is in adownstream direction) and also twists it to convert the longitudinallyextruded ribs into helical ribs. The direction of travel of the belt ofthe puller can relate to the direction of travel of the tube atsubstantially any angle that is desired. This angle is the angleintercepted by a plane that includes the axis of the tube 22 and a planethat includes the centerline of the belt 72. Actually, the centerline ofthe belt 72 and the axis of the tube 22 are skew lines. However, forease of understanding, the plane containing the tube axis and the planecontaining the belt centerline will be referred to as lines having anangle intercepted there between. Assuming that the speed of the belt andthe speed of the tube are in a constant ratio, the angle interceptedbetween the centerline of the belt and the axis of the tube determinesthe angle of the helical rib elements with respect to the axis of thetube. It therefore also controls the pitch and “wave length” of thehelical rib elements. Thus, if the intercepted angle is about 45°, thehelical angle of the ribs will also be about 45°. As the angleintercepted between the axis of the tube and the centerline of the beltincreases, the wave length of the helical ribs shortens wherebyincreasing the circumferential support provided by the ribs between theinner and outer tubular members. This increase in support comes at theexpense of the amount of material in the ribs per linear length of thetube. Conversely, as the angle intercepted between the centerline of thebelt and the axis of the tube is reduced, the wave length and pitch ofthe helical ribs increases. This reduces the amount of material thatmakes up the ribs of this invention per unit of length of the tube. Ifthe orientation of the ribs is more circumferential, the crushresistance of the tubular composite is increased at the expense of thelongitudinal bending resistance of the product. If the orientation ofthe ribs is more longitudinal, the bending stiffness is increased whilethe crush resistance is reduced. While it has been found to be mostconvenient to operate at an angle of about 45° because the design of themachinery is simpler, it is considered to be within the scope of theinstant invention to operate at any angle, especially an angle that isbetween about 10° and 80°, most especially at an angle between about 30°and 60°.

[0101] Referring to FIG. 13, according to this invention, an improvedversion of the previously described puller is set forth. The puller belt72 first comes into contact with the extruded tube 22 at a location 72 athat is determined by the angle of the pulley axis with the axis of thetube. It is preferred that the puller belt is led into contact with thetube surface by an idler pulley 102 at about the same location (distancefrom the idler pulley 106) regardless of the angle at which thecenterline of the belt 72 contacts the surface of the extruded tube 22.The belt 72 is centered on the idler pulley 102 thereby assuring theproper positioning of the belt 72 on the tube 22 and the proper angle ofattack of the belt 72 with regard to the axis of the tube 22. It will beapparent that the angle intercepted between the axis of the pulley 102and the axis of the tube 22 will be complimentary to the angleintercepted between the angle of the tube axis and the centerline of thebelt. With this mathematical relationship, it should be apparent thatthe angle of attack of the belt 72 can be changed by rotating the pulley102 in a plane that includes the pulley axis and is in the direction offlow of the tube 22.

[0102] The belt 72 is fed to the pulley 102 from a fixed idler pulley106 whose axis is preferably disposed normal to the axis of the pulley102. This relationship of these two pulley rolls is effective to twistthe belt 72 a full 90° from its position as it comes off. The spatialrelationship and orientation of the idler pulleys to each other is afunction of the space available in which to position these elements. Aswill be seen below, positioning the idler pulleys 106 and 104 at rightangles relative to the positions of the idler pulley 102 and the drivenpulley 103, respectively, is one important aspect of this invention. Inthe device depicted in FIG. 13, which shows a single belt puller system,the pulleys 102, 104 and 106 are preferably idler pulleys (that is theyare neither independently driven nor do they drive the belt). The pulley103 is the driven pulley.

[0103] The place where the belt 72 leaves the tube 72 b can varydepending on the tube diameter, and on the degree of lateral force thatneeds to be applied to the belt by the tube. After leaving the tube, thebelt enters the long driven roll 103 with its centerline substantiallyperpendicular to the axis of the driven roll 103. The drive roll isdriven by a conventional motor and, if desirable or necessary, a speedreducer (reducing gear) 108. As the belt comes off the driven roll 103,it turns 90° and then is passed around an idler pulley 104. The angle ofthe axis of the idler pulley 104 is normal to the axis of the drivenroll 103 so as to enable it to turn the belt 72 through 90° and therebyalign it with the orientation of the belt as it passes over thepreviously mentioned idler roller 106.

[0104] In the alternative, especially where it is contemplated thatthere will be changes in the diameter of the tube 22, it is consideredto be within the scope of this invention to make the pulley 106 a drivenpulley and the pulley 103 an idler pulley. In addition, it has beenfound to be expedient to position the driven pulley 106 as close to thetube 22 as is practical. In this configuration, the unsupported lengthof the puller belt 72 is made as short as practical and it can bemaintained at that separation regardless of any change in the diameterof the tube 22. It is contemplated that either arrangement will beappropriate in the apparatus of this invention.

[0105] One of the important aspects of this part of this invention,especially where the helical angle of the ribs is 45°, is the employmentof a means to maintain the effective length of the belt the sameregardless of small variations in the diameter of the tube 22 that isbeing pulled/twisted. This is accomplished through a combination ofelements including a tensioner element 107 that acts on the belt 72through a pulley 105. In this aspect of this invention, the idler pulley104 is made adjustable in a direction 109 such that it can be placed inline with where the belt leaves the drive roll 103. The belt 72 twiststhrough an angle of about 90° during its travel from the driven roller103 to the adjustable idler pulley 104. If there are small variations inthe belt position along the drive roller 103, these can be accommodatedby arranging a slight compensating angle in the path from the driveroller 103 to the idler 104.

[0106] The largest variation in belt position along the driven roller103 results from a change in the diameter of the tube 22. For example,if the axis of the product tube is oriented at an angle of 45° withrespect to the center line of the belt as it leaves the surface of thetube 22, an increase of one (1) inch in the diameter of the tube resultsin the belt being positioned 2.2 inches differently along the driveroller. Where the diameter of the tube is changes to a substantialextent, this will be accommodated by a replacement of the puller/twisterbelt for one of longer or shorter length, depending on which way thetube diameter has been changed.

[0107] In a conventional profile extrusion process, where the extrudedtube has not be twisted as well as pulled, the product leaving thepuller is cut to length by a circular saw that is mounted to a movabletable. As the product advances, a clamp on the saw table grips it,making the saw and its table move at the same speed as the product whilethe cut takes place. Then the clamp releases and the saw table returnsto its rest position awaiting the next cut.

[0108] With the multi-walled tubular product of this invention, theconventional cutoff saw is less than completely satisfactory for thefollowing reasons:

[0109] first, the circular saw leaves a somewhat rough cut, and producessawdust, which is undesirable in many applications, such as cores thatare used for film that is used in painting or electronics applications;and,

[0110] second, the advancing product cannot be effective gripped becausethe product is rotating as it advances.

[0111] As shown in FIGS. 10 and 11, in the instant method and apparatusfor producing a helical-rib multi-walled tube of this invention, the cutis made by a knife blade 76 mounted on a hollow ring 78 that spinsaround the tube, suitably at about 350 RPM, while the blade is plungedinto the outer wall of the tube 22 as shown in FIG. 10. This makes adust-free cut on many materials, such as high density polyethylene.

[0112] With higher-stiffness materials like polystyrene, the blade alonecan produce some small cracks or chips. To avoid this, a V-shaped cut ismade in, but not through, the outer wall of the tube 22 before the bladeenters. The mechanism for this consists of a pivoted arm 80 with a Vcutter 82 at the end away from the pivot, and a roller 84 midway alongthe length of the arm. The arm 80 is suitably spring-loaded (not shown)inward against a stop. When the actuating means moves toward the tube22, the blade 82 contacts first, (shown in the FIG. 11) making aV-shaped cut/depression, when the “V” shaped depression is deep enough,the roller 84 contacts the outer wall of the tube whereby limiting thedepth of the cut to less than all the way through the outer tube 22,then the arm pivots about the roller withdrawing the V cutter.

[0113] As shown in FIG. 12, the cutoff machine is on a carriage thattravels with the tube 22 as the cut is being made. On a commercialproduction line, the multi-walled tubular product 22 rotates veryslowly. This makes it necessary to spin the blade and V-cutter 76 and 82about the product. The blade and V-cutter 76 and 82 are mounted to aspinning hollow rotor 90 through which the tubular article 22 passes.When the time arrives to make the cut, a clamp (not shown) grabs theincoming tubular product 22, causing the carriage to ride along with themoving tube. The clamp has wheels, so it allows for the slow rotation ofthe product that was imparted by the helical puller. Then the knife andV-cutter are plunged inward as they spin about the tube. A novel aspectof this cutter machine is a way to reach inside the rotating mechanismto actuate the blade.

[0114] The rotor 90 consists of two large hollow timing belt pulleysside by side. One of the pulleys is driven, by a belt 92, direct fromthe drive motor 94. The second belt 96 is longer, and makes a path abouta series of idlers as it goes around a shuttle 98. As the shuttle ismoved it lengthens the path of the belt 96 on one side of the pulley andshortens it on the other side, so that the second pulley changes itsangular position relative to the first. The blade 76 mount is suspendedbetween the two pulleys, so when the pulleys change angular position,the blade moves inward. In operation, the shuttle is parked at one endof its travel, and the two pulleys rotate at the same speed with theblade retracted outward. When it is time to cut, a linear actuator, suchas an air cylinder (not shown) or linear actuating servo mechanism,moves the shuttle to the other end of its travel, causing the secondpulley to rotate a bit slower and plunging in the blade. Then theshuttle returns to its home end, and the blade retracts, awaiting thenext cut.

[0115] In its rest position, the cutter carriage is held lightly, suchas by an air cylinder, in an upstream direction against a stop. As thetube exits through the cutter, it slides along a table that cradles it,holding it in line. Extending up from this cradle is a tongue that isstruck by the leading edge of the tube. The tongue is attached to thecarriage of the cutter, so the advancing tube pushes on the tongue,causing the carriage to travel downstream along with the tube,overcoming the upstream force of the air cylinder. When the carriagebegins to travel, a sensor releases the air cylinder and begins thecutoff cycle. At the conclusion of the cycle, the tongue retracts, theair cylinder returns the carriage to its rest position, and a pusherfoot pushes the cut portion of the tube sideways onto a slanted tableand into a bin.

[0116] It is considered to be within the scope of this invention thatthe ends of cut lengths of tubular product will be formed into male andfemale configuration so that lengths of tubing can be joined together.These joints can be bell and spigot type joints or threaded joints orthe like.

1. An extruded composite having a helically twisted unitary structure,comprising: a unitary, helically twisted integral assembly of at leastone axially elongated, substantially rigid inner cylindrical memberhaving an outwardly directed wall, at least one axially elongated,substantially rigid outer tubular member, having an inwardly directedwall directed toward and radially spaced from said outwardly directedwall, and a plurality of substantially rigid struts disposed in part ofthe annular space and extending between the inwardly and outwardlydirected walls; wherein at least some of said struts are joined to bothsaid inwardly and outwardly directed walls along a sufficient portion ofthe axial length of said walls to maintain a predetermined radialspacing between said walls; wherein at least some of said struts, thatare paired next adjacent to each other, are disposed at opposite angles,other than perpendicular, with respect to said inwardly directed andsaid outwardly directed walls to which they are joined, and said pairedstruts are joined to said inwardly directed and outwardly directed wallssuch that there is at most a small portion of said wall(s) separatingthe juncture of each of said paired struts with said wall(s); whereinsaid composite structure has an outside diameter that is notsubstantially greater than the outside diameter of a fluid formextrudate from which it was made; wherein said fluid form extrudateinitially comprises an axially aligned, untwisted, moldable, unitarystructure comprising an inner cylindrical member having an outwardlydirected wall, an outer tubular member having an inwardly directed wallradially spaced from, and directed toward, said outwardly directed wall,and angularly disposed strut pairs in supporting contact with both saidinwardly directed wall and said outwardly directed wall; wherein saidfluid form extrudate has been pulled and twisted as a unitary structureto produce moldable helically twisted unitary structure; wherein saidmoldable helically twisted unitary structure has been solidified bycooling from inside said inner cylindrical member and outside said outertubular member into said substantially rigid, helically twisted unitarystructure; and wherein the alignment of said helically twisted haslongitudinal and circumferential alignment components. 2 CANCELLED
 3. Acomposite tube as claimed in claim 1 wherein at least some of saidstruts are simultaneously contacted with and adhered to said inwardlydirected and said outwardly directed walls, respectively, at location(s)that are spaced from the location where the next adjacent struts arecontacted with and adhered to said inwardly and outwardly directedwalls, respectively, so as to form generallylongitudinal/circumferential strut-defining cells having a substantiallytrapezoidal cross section.
 4. A composite tube as claimed in claim 3wherein all of said struts are angularly disposed at alternatingpositive and negative angles away from normal with respect to saidinwardly and outwardly directed walls.
 5. A composite tube as claimed inclaim 1 wherein all of said struts extend the entire longitudinal lengthof said composite tube.
 6. A composite tube as claimed in claim 1wherein said inner cylinder is a hollow tube.
 7. A drainage pipecomprising a plurality of composite tubes as claimed in claim 6 joinedtogether end to end.
 8. A composite tube as claimed in claim 6 having atleast one perforation in at least one of said tubular members.
 9. Adrainage pipe as claimed in claim 7 wherein said composite tubes areperforated.
 10. A composite tube as claimed in claim 6, furthercomprising at least one perforation through said outer tubular member.11. A composite tube as claimed in claim 6, further comprising aplurality of perforations through said inner and outer tubular members,said perforations being positioned such that a volume inside said innertubular member is adapted to communicate with a volume outside saidouter tubular member.
 12. A pipe comprising a plurality of joinedtogether composite tubes as claimed in claim
 11. 13. A septic fieldcomprising a plurality of composite tubes as claimed in claim
 11. 14. Aseptic field comprising a plurality of composite tubes as claimed inclaim 11, wherein some of said composite tubes are joined together endto end.
 15. A method of draining underground water comprising burying atleast one composite tube as claimed in claim 11 proximate to a source ofsaid water and in a position adapted to permit said water to drain bygravity and in a condition adapted to cause underground water to seepinto said inner tube through said perforations, and providing an outletfor said composite tube.
 16. A method of making a helically twistedunitary cylindrical stricture, comprising an outer tubular member, aninner cylindrical member and a plurality of struts disposed in anannular space the between said inner and outer members, and in spacingand supporting relationship to said inner cylindrical member and saidouter tubular member, said method comprising: extruding a unitary,moldable cylindrical profile extrudate, having an outer cross sectionalsize, comprising, as the extrudate, a moldable inner cylindrical member,a moldable outer tubular member and moldable struts in spacing andsupporting contact with said inner cylindrical member and said outertubular member; while in a moldable condition, drawing in a downstreamdirection and twisting, said extrudate passing said twisted and drawnextrudate through a cooling zone, along a twisting path that haslongitudinal and circumferential directional components thatsubstantially correspond to a rate of circumferential twisting and arate of down stream drawing, and cooling said structure from inside saidinner cylindrical member and from outside said outer tubular member anamount sufficient to solidify said unitary extrudate in said drawn andtwisted orientation; downstream of said cooling step, drawing saidsolidified composite unitary structure downstream while simultaneouslytwisting it so as to cause said molten extrudate, at a location betweensaid extrusion and said cooling, to be pulled downstream and twisted,whereby causing said structure to become oriented in both longitudinaland circumferential directions prior to cooling/solidifying it; andcarrying out said method under conditions such that the outer crosssectional size of said solid unitary structure is not substantiallylarger than the outer cross sectional size of said moldable extrudate.17. A method as claimed in claim 16 wherein said inner cylindricalmember is tubular and further comprising drawing said inner tubularmember over a cooling mandrel and through a cooling sleeve underconditions sufficient to cool and solidify said extrudate from theinside as well as from the outside.
 18. A method as claimed in claim 17further comprising cooling and solidifying said inner tubular member,said outer tubular member and said struts at substantially the sametime.
 19. A method as claimed in claim 17 further comprising coolingsaid outer tubular member and proximate portion(s) of said struts byspraying cooling water on an outwardly directed surface thereof.
 20. Amethod as claimed in claim 17 further comprising drawing a vacuumthrough apertures in said cooling mandrel that are adapted to beproximate to an inwardly directed surface of said inner tubular memberan amount sufficient to cause an inwardly directed surface of said innertubular member to maintain close proximity to said mandrel.
 21. A methodas claimed in claim 20 further comprising intermittently releasing saidvacuum whereby permitting said inwardly directed surface to move awayfrom said cooling mandrel for a period of time that is sufficient toprevent said inwardly directed surface from adhering to said mandrelduring said cooling operation.
 22. A method as claimed in claim 16further comprising pulling and twisting said solidified extrudate anamount sufficient to form said moldable extrudate into a substantiallyhelical orientation before it is fully solidified, and solidifying saidtwisted molten extrudate in said helical configuration.
 23. A method asclaimed in claim 22 wherein said inner cylindrical member is tubular andfurther comprising drawing and twisting said solidified extrudate suchthat said cooling extrudate follows a helical path about a coolingmandrel, and rotating said mandrel at a sufficient speed and directionto adjust the rotational speed of said inner tubular member relative tothe rotational speed of said outer tubular member such as to cause saidstruts to not substantially deform prior to and during cooling.
 24. Amethod as claimed in claim 16 further comprising twisting and drawingsaid solidified extrudate by closely contacting a plurality of drivenbelts about an outwardly directed surface of said outer tubular member;wherein different of said belts encounter said outwardly directedsurface from different positions such that bending of said compositearticle is minimized; and driving said belts at a speed sufficient tocause said extrudate to be twisted circumferentially and drawnlongitudinally.
 25. A method as claimed in claim 24 further comprisingcontacting all of said belts with said tubular article at differentpositions.
 26. A method as claimed in claim 24 further comprisingcontacting at least some of said belts with said tubular article atcomplementary positions.
 27. A method as claimed in claim 24 furthercomprising adjusting the length of said belts to compensate for a changein the diameter of said tubular article.
 28. A method as claimed inclaim 16 further comprising feeding said moldable material to saidextruder die from a plurality of feed locations disposed about theperiphery of said extruder die.
 29. A method as claimed in claim 28further comprising feeding said moldable material to said extruder diefrom a plurality of feed locations substantially equally spaced aboutthe periphery of said extruder die.
 30. A method as claimed in claim 28further comprising: A. subdividing a feed of said moldable material intoa plurality of first feed streams; B. subdividing at least some of saidplurality of first feed streams into a plurality of second feed streamsC. repeating step B a sufficient number of times to produce a pluralityof feed streams each of which has a substantially smaller volume thansaid feed; and D. feeding the product of step C substantially evenlydistributed about the periphery of an extrusion die.
 31. A method oftransversely cutting a composite tubular body as claimed in claim 16further comprising: circumferentially applying a “v” shaped cutter aboutsaid composite tube, whereby cutting a “v” groove partially through saidcomposite tube and leaving a web of uncut tubular material; andthereafter, in a separate step, cutting all the way through the web oftubular material.
 32. A method as claimed in claim 31 furthercomprising: axially rotating said tubular body while longitudinallyprogressing said body; causing said “v” cutter to move longitudinally atsubstantially the same speed as said cylindrical body is longitudinallyprogressing: and rotating said “v” cutter while pressing it into saidrotating cylindrical body while rotating said “v” about said cylindricalbody; and pressing said “v” cutter into said rotating cylindrical bodywhereby said rotation of said “v” cutter causes said “v” cutter to cut a“v” groove in said cylindrical body.
 33. A method as claimed in claim 21further comprising, during release of said vacuum, and applying anoverpressure of gas through at least some of said apertures in an amountand at a velocity sufficient to cause said inwardly directed surface tomove away from said mandrel.
 34. A method of producing a compositetubular article which comprises: A. feeding a moldable material throughan extrusion die to form a molten unitary extrudate having a more inwardtubular member, at least one radially spaced apart more outward tubularmember, and a plurality of struts, disposed at angles other thanperpendicular and in supporting contact relationship between said moreinward tubular member and a next adjacent more outwardly spaced aparttubular member; B passing said molten unitary extrudate through acooling zone under conditions sufficient to substantially cool saidextrudate and form a solidified unitary extrudate; C. drawing saidsolidified unitary extrudate in a downstream direction whilesimultaneously twisting the solidified unitary extrudate; wherebycausing said molten extrudate to be twisted circumferentially and bedrawn downstream from said extrusion die into cooling relation with saidcooling zone; whereby causing said molten, twisted, drawn extrudate totraverse said cooling zone along a path that has both longitudinal andcircumferential components and substantially corresponds to the degreeof twisting and downstream drawing imparted to said extrudate; and D.while said tube is progressing downstream along a path havinglongitudinal and circumferential vectors, cutting said tube intodiscrete lengths.
 35. A method as claimed in claim 34 further comprisingforming the opposite ends of at least some of said discrete lengths oftubing into male and mating female profiles, respectively.
 36. Cancelled37. A method as claimed in claim 16 wherein said struts are joined tosaid outer tubular member and said inner cylindrical member,respectively, as they are being extruded.
 38. An extruded compositestructure as claimed in claim 1 wherein at least some of saidalternatingly angularly disposed struts are extruded in simultaneouscontact with and adhered to said inner cylindrical member and said outertubular member, respectively, at location(s) that are adjacent to thelocation where a next adjacent strut is contacted with and adhered tosaid inward cylindrical member and said outwardly directed tubularmember, respectively, whereby said next adjacent, angularly disposedstruts together with the portion of respectively intercepted inwardlydirected or outwardly directed walls of said tubular member and saidcylindrical member, respectively, define cells having a substantiallytriangular cross section.
 39. CANCELLED
 40. Apparatus for forming asubstantially rigid, unitary, cylindrical structure, wherein saidstructure comprises an inner cylindrical member, an outer tubular membersurrounding said inner cylindrical member and radially spaced there fromand a plurality of struts, disposed in an annular space between saidinner and outer members, respectively, in spacing and supportingrelationship to said members; which apparatus comprises: an extruderadapted to simultaneously extrude through a die a unitary moltenextrudate comprising: an inner cylindrical member, an outer tubularmember radially spaced from said inner cylindrical member, and aplurality of said spacing and supporting struts; cooling means disposeddownstream of said extruder die means, comprising: means to cool aninner surface of said extrudate and to solidify said inner cylindricalmember and a portion of said struts adjacent thereto; means to cool anouter surface of said extrudate and to solidify said outer tubularmember and a portion of said struts adjacent thereto; and a pathinscribed on said means to cool said inner cylindrical member adapted tobe followed by said molten extrudate during said cooling, wherein saidcooling means is adapted to freeze said molten extrudate into solidextrudate; and drawing and twisting means downstream of said coolingmeans, and comprising means to twist said solid extrudate in acircumferential direction and means to draw said solid extrudate in adownstream, axial direction; wherein operation of said drawing andtwisting means is adapted to be operative to draw and twist saidsolidified extrudate and to thereby cause said molten unitary extrudateto be twisted and drawn into a configuration having a circumferentialcomponent and a longitudinal component; and wherein said path isconsistent with the twisted and drawn configuration of said solidifiedextrudate.
 41. The apparatus as claimed in claim 40 wherein said path issubstantially a helix.
 42. The apparatus as claimed in claim 40 whereinsaid drawing and twisting means are a plurality of belt means adapted tobe in frictional, helical contact with a sufficient portion of thecircumference of said solidified extrudate such that said pulling andtwisting are accomplished substantially simultaneously.
 43. Theapparatus as claimed in claim 40 wherein said inner member is tubularand wherein said cooling means further comprises: a cooling mandreladapted to fit within said inner tubular member; a cooling sleeveadapted to substantially surround said outer tubular member; means toapply cooling from said mandrel in an amount sufficient to solidify saidinner tubular member and at least a portion of said struts that areproximate to said inner tubular member; and means to apply cooling fromsaid sleeve in an amount sufficient to solidify at least said outertubular member and at least a portion of said struts that are proximateto said outer tubular member.
 44. The apparatus as claimed in claim 43wherein said mandrel is tapered in a downstream direction and said taperis at an angle that corresponds to an amount of shrinkage that saidinner tubular member will realize in passing through said cooling zone.45. The apparatus as claimed in claim 43 wherein said mandrel comprisesapertures in a surface that is adapted to be proximate to said innertubular member and further comprising vacuum drawing means operativelyassociated with said apertures in an amount sufficient to maintain saidinner tubular member in close proximity to said cooling mandrel duringat least a portion of said cooling.
 46. The apparatus as claimed inclaim 45 further comprising means to periodically relieve said vacuum ata frequency such that said inner tubular member will not permanentlyadhere to said mandrel.
 47. The apparatus as claimed in claim 46 furthercomprising means to intermittently release said vacuum and exertoutwardly directed pressure through said apertures in an amount andfrequency such as to cause said inner tubular member to intermittentlymove away from said mandrel.
 48. CANCELLED.
 49. An apparatus for coolingand solidifying a tubular extrudate comprising: a mandrel adapted tosupport a molten tubular extrudate about a surface thereof; apertures insaid surface that are adapted to be proximate to said an inside surfaceof said tubular extrudate; means to apply cooling to said surface andvacuum drawing means, operatively associated with said apertures,adapted to draw sufficient vacuum through said apertures to maintainsaid tubular extrudate in close proximity to said cooling mandrel duringat least a portion of said cooling.
 50. The apparatus as claimed inclaim 49 further comprising means to periodically relieve said vacuum ata frequency that is adapted to prevent said molten extrudate frompermanently adhering to said mandrel.
 51. The apparatus as claimed inclaim 50 further comprising means to intermittently release said vacuumand exert outwardly directed pressure through said apertures in anamount and frequency that is adapted to cause said molten extrudate toperiodically move away from said mandrel.
 52. The apparatus as claimedin claim 50 wherein said vacuum relief is accomplished at a frequency ofabout 100 to 400 times per minute.
 53. An apparatus for pulsing apressure change to a housing comprising: an upper plate having anaperture there through; a lower plate having an aperture there through;wherein said apertures are adapted to have overlapping cross sections;and a disk that is adapted to be disposed between said upper plate andsaid lower plate; wherein said disk has a slot there through disposedproximate to, and radially internal of, a peripheral edge of said disk;said slot passing through the entire thickness of said disk; and ends ofsaid slot being separated by a solid portion of said disk of a size thatis sufficient to be interposed between said apertures; wherein saidoverlapping cross sections of said apertures are adapted to be alignedwith each other and with said slot; wherein said plates are rotatable inrelation to said disk such that said apertures are adapted tocommunicate with each other through said slot and are adapted to notcommunicate with each other when they are separated by said solidportion; and wherein the rate of rotation of said plates relative tosaid disk determines the pulse rate.
 54. The apparatus as claimed inclaim 53 further comprising vacuum drawing means operatively associatedwith one of said apertures and said housing being operatively associatedwith the other of said apertures.
 55. The apparatus as claimed in claim54 further comprising pressure generating means operatively associatedwith the aperture with which said vacuum drawing means is associated andswitching means adapted to alternately apply vacuum and pressure to saidhousing through said apertures.
 56. The apparatus as claimed in claim 55wherein said solid portion of said disk further comprises a recessedportion, facing a first of said plates, so configured that when saidsolid portion of said disk is interposed between said plates, there is apassageway from one end of the aperture in said first plate through saidaperture and thence through space between said first plate and saidrecessed portion; and wherein there is substantially no passagewaybetween said solid portion and the aperture in said second plate.
 57. Amethod of producing a cylindrical shaped object comprising a tubularmember, said method comprising: extruding a tubular profile, moldableextrudate from an extruder, while in a moldable condition, causing saidtubular extrudate to be twisted circumferentially and drawn down streamfrom said extruder; cooling said molten extrudate by passing it througha cooling zone along a twisting path that has longitudinal andcircumferential directional components that substantially correspond toa rate of circumferential twisting and a rate of down stream drawing,and cooling said structure an amount sufficient to solidify it; anddownstream of said cooling step, drawing said solidified compositestructure downstream and twisting it so as to cause said moltenextrudate, at a location between said extruding and said cooling, to bepulled downstream and twisted, as aforesaid, whereby causing saidstructure to become oriented in both longitudinal and circumferentialdirections prior to solidifying it.
 58. The method as claimed in claim57 wherein said downstream drawing and twisting are carried outsubstantially simultaneously.
 59. The method as claimed in claim 57further comprising drawing and twisting said moldable extrudate anamount sufficient to convert it into a helically shaped tube.
 60. Themethod as claimed in claim 57 further comprising cooling and solidifyingsaid twisted extrudate from both the inside and the outside of saidtwisted molten tubular extrudate.
 61. The method as claimed in claim 57further comprising extruding said molten extrudate comprising an innertubular member and an outer tubular member.
 62. The method as claimed inclaim 57 further comprising drawing a vacuum through apertures in saidcooling mandrel sufficient to maintain said tubular extrudate proximateto and in cooling relationship with, said cooling mandrel.
 63. Themethod as claimed in claim 62 further comprising periodically stoppingthe vacuum being drawn through said mandrel apertures, and therebypermitting said tubular extrudate to move away from said coolingmandrel.
 64. The method as claimed in claim 63 further comprising,during the time that the vacuum is not being drawn through said coolingmandrel apertures, applying positive pressure through said coolingmandrel apertures, and thereby forcing said tubular extrudate away fromsaid cooling mandrel.
 65. A method of producing a shaped objectcomprising a cylindrical member, said method comprising: extruding acylindrical profile, moldable extrudate from an extruder, while in amoldable condition, causing said cylindrical extrudate to be twistedcircumferentially and drawn down stream from said extruder; passing saidextrudate through a cooling zone, along a twisting path that haslongitudinal and circumferential directional components thatsubstantially correspond to a rate of circumferential twisting and arate of down stream drawing, and cooling said structure an amountsufficient to solidify it; and downstream of said cooling step, drawingsaid solidified composite structure downstream and twisting it so as tocause said molten cylindrical extrudate, at a location between saidextruding and said cooling, to be drawn downstream and twisted, asaforesaid, whereby causing said structure to become oriented in bothlongitudinal and circumferential directions prior to solidifying it;wherein said drawing and twisting operations are carried outsubstantially simultaneously and said substantially simultaneous drawingand twisting cause the resultant cylindrical profile article to have agreater uniformity of cross section and a greater straightness thanwould a cylindrical profile that had not been made using saidsubstantially simultaneous drawing and twisting 66 A method as claimedin claim 65 further comprising drawing and twisting said extrudate bywrapping a plurality of moveable belts longitudinally spaced from eachother and disposed helically about, and in frictional contact with, saidsolidified cylindrical profile; and moving said belts in a helicaldirection relative to the axis of said cylindrical profile underfrictional conditions sufficient to cause said cylindrical profile to bemoved in a helical direction relative to its axis. 67 An extrusionapparatus comprising: an extruder barrel having: at least one apertureadapted to have extrudable material fed therethrough, means to heat saidfeed material to a molten condition; and means to move molten feedthrough said barrel; an extruder die; an exit conduit from said extruderbarrel adapted to convey molten feed material downstream of saidextruder barrel and through said extruder die; means between said barreland said die adapted to subdivide molten feed into a plurality of moltenstreams; means to subdivide said subdivided molten streams into anadditional plurality of molten streams; and a plurality of conduitsadapted to convey said plurality of molten streams to a die assembly;wherein said extruder die assembly comprises a die aperture operativelycommunicating with a plurality of said conduits that are adapted tocontain said plurality of molten streams, wherein said die aperturecomprises: an inner arcuate die aperture portion, an outer arcuate dieaperture portion radially spaced from said inner die aperture portion,and a plurality of radially disposed die aperture portions incommunicating relationship with said both said inner and outer arcuatedie aperture portions.
 68. An extrusion apparatus as claimed in claim 67further comprising a plurality of extruder die conduit means throughsaid extruder die assembly communicating between ambient atmosphereoutside said extrusion apparatus and areas of said extruder die disposedbetween next adjacent radially disposed die apertures, and between saidinner arcuate due aperture and said outer arcuate die aperture; whereinsaid extruder die conduit means are adapted to pass gas into regionsbetween extrudate streams emerging from said channel die apertures andto thereby assist in maintaining open, longitudinal cells between bynext adjacent extrudate streams exiting from said inner arcuate dieaperture portion, said outer arcuate die aperture portion and proximateradially disposed die aperture portions.
 69. An apparatus adapted tosubstantially simultaneously draw a work piece down stream and twist thework piece about a longitudinal axis of said work piece, said apparatuscomprising: a plurality of substantially endless belts adapted to befrictionally disposed helically about said work piece in non overlappingrelationship; and means to independently move each of said plurality ofbelts in the same direction and at substantially the same rate of speed,whereby enabling said belts to frictionally engage said work piece andto thereby twist said work piece circumferentially and substantiallysimultaneously draw said work piece downstream.
 70. An apparatus asclaimed in claim 69 comprising two said belts.
 71. An apparatus asclaimed in claim 69 wherein said belts are adapted to be longitudinallynon-overlapping where they contact said work piece.
 72. An apparatus asclaimed in claim 69 wherein said work piece is cylindrical in profileabout a longitudinal axis; and wherein each of said belts is adapted toengage the surface of said work piece along an area that extends from aline segment where the belt first intercepts the surface of the workpiece to a line segment where the belt last intercepts the surface ofthe work piece, and wherein said line segments are parts of the sameline in a direction axial to said work piece.
 73. An apparatus asclaimed in claim 69 wherein said means to move said belts comprises adriven drawing roller spaced from said work piece, wherein said belt isadapted to be wrapped at least partially around said drawing roller andis in sufficient frictional contact with said drawing roller not to slipto any substantial extent when in an operative relationship to said workpiece.
 74. An apparatus as claimed in claim 73 further comprising asmall diameter, first idler roller adapted to be in operativerelationship with said belt and adapted to be disposed downstream ofsaid drawing roller such that the axis of said first idler roller isdisposed at an angle of about 90° with respect to the axis of saiddrawing roller, wherein, when in operation, said belt is adapted tofirst contact said drawing roller and then contact said first idlerroller and wherein said drawing and first idler rollers cooperate tocause said belt to turn substantially 90° between leaving said drawingroller and entering said first idler roller.
 75. An apparatus as claimedin claim 74 wherein the axis of said drawing roller is disposed at anangle with respect to the axis of said work piece such as to maintain ahelical angle between said belt and said work piece.
 76. An apparatus asclaimed in claim 75 further comprising a second idler roller disposeddownstream of said first idler roller, wherein said belt is adapted tobe in operative contact with both of said idler rollers, and wherein theaxes of said first and second idler rollers are substantially coplanar.77. An apparatus as claimed in claim 75 further comprising at least onetensioning means disposed in operative relationship to said belt.
 78. Anapparatus as claimed in claim 76 further comprising a lead in rollerdisposed between said second idler roller and said work piece, whereinsaid belt is adapted to be in operative contact with said second idlerroller, said lead in roller and thence said work piece.
 79. An apparatusas claimed in claim 78 wherein the axis of said lead in roller is insubstantially the same plane as the axis of said drawing roller, wherebybeing adapted to lead said belt onto said work piece at substantiallythe same angle, with respect to the axis of the work piece, as said beltleaves said work piece toward said drawing roller.
 80. An apparatus asclaimed in claim 79 wherein the plane of the axes of said first andsecond idler rollers is transverse to the plane of the axes of said leadin and drawing rollers.
 81. An apparatus as acclaimed in claim 80wherein said belt is endless.
 82. An apparatus as claimed in claim 81comprising two belts adapted to be disposed in an axial nestingarrangement on the surface of said work piece; and wherein each of saidbelts is adapted to be operatively associated with a separate drawingmeans, drawing roller, idler rollers and lead in roller.
 83. A compositetube as claimed in claim 1 that is seamless.
 84. A composite tube madeby the method of claim
 16. 85. A composite tube made by the method ofclaim 53.